1. Field of the Invention
The invention relates in general to a method of fabricating a device isolation structure, and more particularly, to a method of fabricating a shallow trench isolation (STI) structure with an enhanced stability of endpoint detection.
2. Description of the Related Art
For the well-developed integrated circuits, to further shrink the device and increase the integration are the consequent orientations and important topics for industry. As the device dimensions decrease, the integration continuously increases, the difficulty of device isolation technique becomes more significant. The field oxide formed by local oxidation (LOCOS) is restricted for further shrinkage due to its bird""s beak characteristics. Shallow trench isolation structure are thus developed and widely applied, especially in the sub-half micron integrated circuit fabrication process.
In the conventional method of fabricating the shallow trench isolation structure, a pad oxide layer and a silicon nitride mask layer are formed on a substrate first. A steep trench is then formed in the substrate using anisotropic dry etching. The trench is filled with an insulation layer. The excess insulation is removed using chemical-mechanical polishing, such that the remaining insulation layer in the trench is used as the device isolation structure. The mask layer and the pad oxide layer are then removed.
In the conventional method described above, no method related to the stability of endpoint detection has been proposed. As the device dimension shrinks and the integration grows, the silicon nitride mask layer used as the polishing endpoint is becoming much thinner. The stability of endpoint detection is thus easily affected to cause the decreased reliability.
The invention provides a method of fabricating a shallow trench isolation structure with an enhanced stability of endpoint detection. A pad oxide layer, a mask layer, a dielectric anti-reflection coating (DARC) layer and a cap oxide layer are formed on a substrate. A patterned photoresist layer is formed on the cap oxide layer. Using the patterned photoresist layer as a mask, the cap oxide layer, the dielectric anti-reflection layer, the mask layer, the pad oxide layer, and the substrate are etched to form a trench in the substrate. The patterned photoresist layer is removed. A liner oxide layer is formed along a surface of the trench. An insulation layer is formed over the substrate to fill the trench. Using the mask layer as a polishing endpoint, a chemical-mechanical polishing step is performed to remove the excess insulation layer, the cap oxide layer and the dielectric anti-reflection coating layer over the mask layer. The thickness of mask layer is controlled within a first fixed range, while the dielectric anti-reflection coating layer is controlled within a second fixed range. Thus, a light source used in an optical endpoint detection system has the maximum reflected light signal. The mask layer and the pad oxide layer are then removed.
To achieve the objective of stablizing the endpoint detection, the invention accommodates the structure of the mask layer and the dielectric anti-reflection coating layer, that is, by controlling the thickness of the mask layer and the dielectric anti-reflection coating layer within a specific range. The substrate reflectivity for the endpoint detection is thus increased. The larger the substrate reflectivity is, the higher the reliability of the endpoint detection is. Therefore, the objective of having the stable endpoint detection for the fabrication process of the shallow trench isolation structure can be achieved.
Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.